The present invention relates to a projection data correction method and apparatus and a radiation tomographic imaging method and apparatus, and more particularly to a method and apparatus for individually correcting data constituting a projection of an object to be imaged by. penetrating radiation, and a radiation tomographic imaging method employing such a projection data correction method and a radiation tomographic imaging apparatus, comprising the projection data correction apparatus.
In a radiation tomographic imaging apparatus employing an X-ray, i.e., an X-ray. CT (computerizes tomography) apparatus, an X-ray beam having a width containing a range to be imaged and having a thickness in a direction orthogonal to the width is emitted from an X-ray emitting apparatus. The thickness of the X-ray beam can be changed by regulating the openness of an X-ray passing aperture of a collimator, and the slice thickness for imaging is thus adjusted.
An X-ray detecting apparatus comprises a multi-channel X-ray detector to detect an X-ray, and the detector has a multiplicity (e.g., of the order of 1,000) of X-ray detector elements arranged in an array in the direction of the X-ray width.
The X-ray emitting/detecting apparatus is rotated (or scans) around an object to be imaged to generate X-ray projection data of the object in a plurality of view directions around the object, and a tomographic image is produced (or reconstructed) by a computer based on the projection data.
The SNR (signal-to-noise ratio) of the projection data is varied with the xe2x80x98countxe2x80x99 of the penetrating X-ray, and a smaller count gives a lower SNR, if the object to be imaged provides significantly different counts, i.e., significantly different SNR""s of the projection data, depending on the view direction, the reconstructed image suffers from false linear images, or so-called streaking artifacts, due to the effect of a noise pattern in the direction which provides a poor SNR.
Thus, when the thorax or abdomen is imaged with both the arms put on the sides of the body, for example, the streaking artifacts inevitably appear in the direction which connects the arms, leading to a problem that an image having a good quality cannot be obtained.
The present invention is directed to solving the aforementioned problem, and the object of the invention is to provide a projection data -correction method and apparatus that prevents artifact generation in the direction which provides a low count of penetrating radiation, and a radiation tomographic imaging method employing such a projection data correction method and a radiation tomographic imaging apparatus comprising the projection data correction apparatus.
(1) In accordance with a first aspect of the invention to solve the aforementioned problem, there is provided a projection data correction method comprising the steps of: individually calculating an average value of data and its proximate data, the data constituting a projection of an object to be imaged by penetrating radiation; calculating a first product by multiplying the data by a first weighting factor that corresponds to a count of the penetrating radiation from which the data is derived; calculating a second product by multiplying the average value by a second weighting factor that is a complement of the first weighting factor with respect to one; and adding the first and second products.
(2) In accordance with a second aspect of the invention to solve the aforementioned problem, there is provided a projection data correction apparatus comprising: average value. calculating means for individually calculating an average value of data and its proximate data, said data constituting a projection of an object to be imaged by penetrating radiation; first product calculating means for calculating a first product by multiplying said data by a first weighting factor that corresponds to a count of the penetrating radiation from which said data is derived; second product calculating means for calculating a second product by multiplying said average value by a second weighting factor that is a complement of said first weighting factor with respect to one; and adder means for adding said first and second products.
(3) In accordance with a third aspect of the invention to solve the aforementioned problem, there is provided a radiation tomographic imaging apparatus comprising: radiation emitting/detecting means for. emitting radiation toward an object to be imaged in a plurality of view directions and detecting penetrating radiation, and image producing means for producing an image using a projection based on signals detected by said radiation emitting/detecting means, wherein said image producing means comprises: average value calculating means for individually calculating an average value of data and its proximate data, said data constituting said projection; first product calculating means for calculating a first product by multiplying said data by a first weighting factor that corresponds to a count of the penetrating radiation from which said data is derived; second product calculating means for calculating a second product by multiplying said average value by a second weighting factor that is a complement of said first weighting factor with respect to one; adder means for adding said first and second products; and image reconstruction means for reconstructing an image using a projection comprised of data resulted from said addition.
(4) In accordance with a fourth aspect of the invention to solve the aforementioned problem, there is provided the radiation tomographic imaging apparatus as described in (3), wherein an X-ray is employed as the radiation.
(5) In accordance with a fifth aspect of the invention to solve the aforementioned problem, there is provided the above-described projection data correction method, wherein the first weighting factor decreases as the corresponding count becomes smaller.
(6) In accordance with a sixth aspect of the invention to solve the aforementioned problem, there is provided the above-described projection data correction method, comprising the step of employing data obtained by a scout scan or data of a view of an axial scan to determine a rate at which the first weighting factor decreases as the corresponding count becomes smaller.
(7) In accordance with a seventh aspect of the invention to solve the aforementioned problem, there is provided the above-described projection data correction apparatus, comprising first weighting factor determining means for determining the first weighting factor so that the first weighting factor decreases as the corresponding count becomes smaller.
(8) In accordance with an eighth aspect of the invention to solve the -aforementioned problem, there is provided the above-described projection data correction apparatus, wherein the first weighting factor determining means employs data obtained by a scout scan or data of a view of an axial scan to determine a rate at which the first weighting factor is decreased as the corresponding count becomes smaller,
(9) In accordance with a ninth aspect of the invention, to solve the aforementioned problem, there is provided the above-described radiation tomographic imaging apparatus, comprising first weighting factor determining means for determining the first weighting factor so that the first weighting factor decreases as the corresponding count becomes smaller.
(10) In accordance with a tenth aspect of the invention to solve the aforementioned problem, there is provided the above-described radiation tomographic imaging apparatus, wherein the first weighting factor determining means employs data obtained by a scout scan or data of a view of an axial scan to determine a rate at which the first weighting factor is decreased as the corresponding count becomes smaller.
In any one of (1)-(10), it is preferred that the first weighting factor be given by the following equation:
First weighting factor=1xe2x88x92exp {xe2x88x92C/xcex1},
Insofar as artifacts are effectively prevented, wherein
C: a count of the penetrating radiation from which the data is derived, and
xcex1: a constant determined by the area of the projection.
It is preferred that the constant a be determined employing data obtained by a scout scan or data of a start view of an axial scan, and finding a value of the constant a from the area of the projection of that data by an equation or a table.
(Effect)
According to the present invention, the projection: data is individually corrected by a weighted addition with an average value containing the proximate data. The weight of the data is made to correspond to the count of penetrating radiation from which the data is derived. Thus, data with a lower count has a smaller weight of that data and a greater weight of the average value, which can compensate for reduction in SNR.
Moreover, according to the present invention, when the weight of data is decreased and the weight of an average value is increased as the count becomes smaller, a rate at which the weight of data is decreased and a rate at which the weight of an average value is increased as the count becomes smaller are determined employing data obtained by a scout scan or data of a view of an axial scan. Thus, the correction can be relatively weakened by reducing the rate when the size of the object to be imaged is large and streaking artifacts are relatively avoided (i.e., when the difference in SNR of projection data depending on the view direction is relatively small); and the correction can be relatively strengthened by increasing the rate when the size of the object to be imaged is small and streaking artifacts relatively tend to appear (i.e., when the difference in SNR of projection data depending on the view direction is relatively large). Accordingly, a suitable correction can be made regardless of the size of the object to be imaged.
Thus, the present invention can provide a projection data correction method and apparatus that prevents artifact generation in the direction which provides a low count of penetrating radiation, and a radiation tomographic imaging apparatus comprising such a projection data correction apparatus.